Standard NTSC television scans 525 lines per frame in the form of two sequential fields of 2621/2 lines each. The lines of each field interlace with the lines of adjacent fields and the eye of a viewer integrates these interlaced lines to reduce the effect of 30 Hz frame-rate flicker. However, the line structure is still visible under certain circumstances, and is particularly visible on large-screen television displays viewed from a relatively close distance. The problem is made even more severe by the ultra-large pictures formed by projection-type television displays. The advantages of such ultra-large pictures in providing the illusion of surrounding the viewer are reduced by the need for the viewer to remain sufficiently far from the display so that the line structure is not so visible.
A compatible high-definition television system is described in U.S. patent application Ser. No. 288,753 filed July 31, 1981, in the name of C. B. Oakley et al. which issued Jan. 31, 1984 as U.S. Pat. No. 4,429,327. In this system, the visibility of the line structure is reduced in a manner compatible with standef NTSC (or PAL) television by using a camera which generates two lines for every standard line (for example, 1,050 lines-per-frame rather than 525), forming separate signals related to the sums and differences of pixels on adjacent raster lines, and transmitting as a compatible signal the sum signal, together with the difference signal, which may be transmitted separately or concealed within a composite color signal. This arrangement increases the vertical resolution, by increasing the number of lines, which makes it possible to view an ultra-large picture from a closer distance without discerning the line structure. With this system, the vertical luminance and chrominance resolution becomes about 1,000 lines, while the horizontal resolution, which is established by the luminance bandwidth, remains at about 240 television lines. The horizontal resolution thus becomes the limiting factor in the distance between the viewer and an ultra-large display, once the vertical line structure becomes invisibile.
Other compatible high-definition television systems are described in U.S. patent application Ser. Nos. 352,001 and 424,232, filed Feb. 24, 1982, and Sept. 27, 1982, in the names of R. N. Hurst and K. H. Powers, respectively. In these systems the scanning spot in the camera is wobbled in a zigzag pattern to double the resolution in both the horizontal and vertical directions of the high-definition display. The wider bandwidth signal which is transmitted is compatible with standef television receivers; the effect of the narrow bandwidth of such receivers is to average adjacent pixels in both horizontal and vertical directions. In the high-definition wider bandwidth television receiver, the scanning spot is synchronized to wobble in accordance with the wobble that was introduced by the camera or, in one embodiment, to progressively scan. A disadvantage of spot wobbling is that certain scanning artifacts on the television display may become visible. In these systems the scanning line structure may display a visible high-frequency modulation of the space between adjacent lines of the same field which will appear to move in a way that may be objectionable to the viewer.